It was the end of Kate's first semester of college. She and her roommate, Mallory, decided to skip their final classes on Friday. Instead, they drove to the coast to stay with Mallory's father for the weekend before their final exams week started on Monday. A wealthy older doctor, Mallory's father lived in a large, expensive beach house on a secluded stretch of pristine coastline. It was the perfect place to relax and recuperate before the grueling week they had ahead of them.

He wasn't home when they first arrived, so Kate and Mallory let themselves into his house and started to get ready for bed. It was late, and...